Electroluminescent devices based on organic thin layers are light-emitting devices similar to semiconductor-based light-emitting diodes, and are currently being considered for the next generation of flat panel displays. Structurally, these devices contain spaced electrodes separated by an electroluminescent medium which emits light in response to the application of an electrical potential difference across the electrodes.
Preferred forms of OLEDs typically include an anode, an organic hole injecting and transporting zone in contact with the anode, an electron injecting and transporting zone forming a junction with the organic hole injecting and transporting zone, and a cathode in contact with the electron injecting and transporting zone. By application of an electric potential across the electrodes, holes and electrons are injected into the organic zones from the anode and cathode, respectively. Light emission results from the hole-electron recombination within the subject device. This carrier recombination generates excited molecules, which eventually emit light or become thermally deactivated. The efficiency of present OLEDs is thus highly dependent upon both carrier recombination efficiency and photoluminescence quantum yield of the emitting material.
In OLEDs based on small molecules, one of the common electroluminescent units is a highly flourescent aluminum complex, tris(8-quinolinolato)aluminum(III) (Alq3). This flourescent aluminum complex (Alq3) emits a green light and fulfills a number of prerequisites, including but not limited to, when used the device is stable and is quite suitable for carrier transport. The luminescence yield, however, is relatively low. Attempts to improve the luminescence yield have been made by doping the Alq3 layer with fluorescent dye molecules.
Kido et al., Appl. Phys. Lett., vol. 73, no. 19, pages 2721-2723, dated Nov. 9, 1998, discloses an efficient organic electroluminescent device which is fabricated by using tris(4-methyl-8-quinolinolato)aluminum(III) (Almq3) as an emitter layer. Additionally, to using this complex, a multi-layer device structure consisting of a hole-injecting layer, a hole transport layer, a dye-doped Almq3 emitting layer, and an electron transport layer was employed in order to reduce the driving voltage as well as to maximize carrier recombination efficiency. Kido et al. reports a maximum luminescence of over 140 000 cd/m2 and an external quantum efficiency of 7.1%, which is believed to be the highest efficiency ever reported for organic devices.
U.S. Pat. No. 5,150,006, discloses an internal junction organic electroluminescent device comprised of, in sequence, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone, and a cathode. The organic electron injecting and transporting zone is comprised of an electron injecting layer in contact with the cathode. Interposed between the electron injecting layer and the organic hole injecting and transporting zone is a blue emitting luminescent layer comprised of an aluminum chelate containing a phenolato ligand and two Rs-8-quinolinolato ligands, where Rs substituents are used to block the attachment of more than two substituted 8-quinolinolato ligands to the aluminum atom. The presence of the phenolato ligand shifts the device emission to the shorter blue wavelengths of the spectrum and increases emission efficiency. Increased operating stability can be realized by the incorporation of a pentacarboxyclic aromatic fluroescent dye.
U.S. Pat. No. 5,456,988 discloses an electroluminescent (EL) device including an organic electron transport layer comprising Alq3 substituted with Cl or Br in the 5-position. A useful EL device is provided that has excellent durability and retains stable luminescence for a long period of time by using a compound other than the 8-quinolinolato-aluminum complex as an emitting material. No values of luminescence for the single halogen substitution, however, are reported.
A need has thus been recognized to enhance the luminescence of OLEDs.